No, the large indents are the gear selections, and I've redone your pic and there should be a small notch at the other red arrow on the lump on the left between first & second; those lower bits are where the indent plunger locates ( for some reason someone put a second one in the position of the yellow arrow on a camplate I have- I think they wanted a reverse camplate, but think there may be more to it than that )

Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easyAustralia

JulianS, Dutch, thank you very much. Haynes (God bless him) mentions that it is the shallow indentation, but when I postioned it in the box it did not look right. For some reason I got it into my head that the flat straight outside edge should be postioned top most horizontally. I know, I know, I should have checked before I removed it. I will put it all back together again and this time maybe it will function as BSA intended.John

Since posting this topic, I have been skimming through D W Munro's book BSA Twin Motorcycles, and lo and behold, look what I discovered in the process. Alright it is a plunger box, and mine is a swing arm, but none the less it shows the correct orientation of the camplate. I must say that it never crossed my mind that I already had the answer to my post tucked away in this book. For those who are not aware, D W Munro was a member of the A10/A7 design and developement team.John